In a combined gas and steam turbine plant the heat contained in the expanded operating medium or flue gas is used for evaporating a flow medium, usually water. The (water) steam thus produced is then used to drive a steam turbine. The heat is transferred in such cases in a waste heat steam collection drum or waste heat steam generator connected downstream on the flue gas side of the gas turbine, in which heating surfaces are arranged in the form of pipes or bundles of pipes in which the flow medium to be evaporated is conducted. These heating surfaces are usually a component of a flow medium circuit which also comprises the steam turbine and the condenser connected downstream from it, e.g. a water steam circuit, with the expanded flow medium exiting from the steam turbine being directed after its condensation in the condenser back again to the heating surfaces of the waste heat steam generator. As well as the evaporator heating surfaces, further heating surfaces can also be provided in the waste heat steam generator, especially for preheating the condensate or feed water or for superheating the generated steam. Furthermore a supplementary firing facility can also be integrated into the waste heat steam generator, for example an oil firing facility, in order to either raise the temperature of the flue gas above the level at its exit from the gas turbine, or with a decoupled or shut down gas turbine, to still be able to maintain the steam production in the waste heat boiler (so-called oil operation).
Usually the flow medium circuit comprises a number, for example three, pressure stages, each with its own evaporator section. A proven construction and design concept for such an evaporator section because its structure is kept comparatively simple and its relative ease of operation is based—at least in the area of less than critical steam pressures—on the natural circulation principle. In such cases a steam collection drum arranged above the flue gas flow channel of the waste heat steam generator, which is sometimes also referred to as the “top drum” serves as a reservoir for the preheated condensate or feed water arriving from the condensate or feed water pump, where necessary through a condensate pre-heater or an economizer. During operation a part of the stored water sinks downwards, driven by its own weight or by the hydrostatic pressure of the water column continuously through downpipes connected to the floor or sump of the steam collection drum. Via an intermediate distribution collector which is occasionally also referred to as the “bottom drum” the water which has dropped down is distributed to a number of riser pipes connected in parallel and bundled into heating surfaces heated by the heat contained in the flue gas and/or by the radiation heat generated by the additional burner of the waste heat boiler, in which the desired evaporation occurs. The heating surfaces formed from the riser pipes can in this case be part of the surrounding wall of the waste heat boiler or be arranged in the manner of bulkhead heating surfaces within the flue gas flow channel surrounded by the surrounding wall.
Because of its reduced density in relation to the liquid aggregate state, the water-steam mixture generated in the riser pipes by (part) evaporation of the water rises upwards and eventually arrives above the liquid level back in the steam collection drum, whereby the evaporator circuit is completed. The water-steam separation, also referred to as phase separation occurs in the steam collection drum; The water vapor present above the water level under saturated steam conditions is fed via a steam discharge pipe connected to the head of the steam collection drum and after superheating where necessary for its further use, e.g. for driving a steam turbine.
The evaporator stages based on the forced circulation principle are similarly constructed, but also feature a circulation pump connected into the evaporator loop which supports or forces the circulation of the water or of the water-steam mixture.
Because of the limited thermal load capability of the pipe wall materials usually used for the heating pipes or riser pipes in the prior art of science and technology it has been necessary to make absolutely sure that during operation of a gas and steam turbine plant the above-mentioned type of riser pipes of the respective evaporator stage are supplied in all operating states sufficiently with flow medium, as a rule water or water-steam mixture. The aim in such cases is to ensure a certain minimum cooling of the pipe walls as a result of the heat transfer from the internal pipe wall surface to the partly evaporating flow medium in this case and thus to avoid any damage to the evaporator circuit and any associated operating risks. In other words: A so-called dry operation of the evaporator or an operation with a reduced water level in which the column of liquid in the steam collection drum and in the downpipes connected to it sinks below a level of the connection of the downpipes or even the downpipes and the riser pipes connected downstream from them are operated completely “dry”, so that practically no flow medium is flowing through them any more, is to be avoided under all circumstances.
These types of consideration are also the basis for the previously employed internationally valid regulations DIN EN 12592 which apply in accordance with part 1 to “water boilers with a volume of more than 2 liters for generating steam and/or hot water with a permitted pressure of more than 0.5 bar and a temperature of over 110° C. and which in accordance with part 7 defines the permitted lowest water level in the steam collection drum as “150 mm above the highest heated point of the drum and the highest connection of the downpipes (upper edge) to the boiler”. The internationally valid successor standard DIN IEC 61508 and DIN IEC 61511 introduced into Germany in the year 2002 does not contain these types of detailed specifications explicitly any longer but the security requirements specified therein have not diminished in any way despite the more flexible framework specifications.
To enable adherence to the said minimum fill levels of liquid flow medium in the steam collection drum with rapid changes in load of the waste heat steam generator for example or with an unforeseen interruption or disconnection of the feed water supply as a result of faults and in order especially in the last mentioned case to be able to dissipate the residual heat present in the system in a safe and material-protective way, the volume of the steam collection drum and the quantity of flow medium retained in it in normal operation (feed water) is usually dimensioned comparatively large, taking into account a “safety margin”. This is however associated with a correspondingly high manufacturing outlay and thereby also with high manufacturing costs.
In accordance with the particular relevance attributed to adhering to the minimum water level in the steam collection drum, in existing plants there is also a three-fold redundant measurement or monitoring of the current fill level related to the drum or to the upper edge of the downpipes, which requires a relatively expensive design of the associated safety facilities. As soon as a two-out-of-three selection from the three level measurements signals a fall in the water level to below a predefined limit value, e.g. 150 mm in accordance with DIN EN 12952, the safety system suppresses any further supply of the hot gas turbine exhaust gases into the waste heat steam generator, e.g. by rapid deactivation of the gas turbine or by operating a corresponding valve the waste gases are diverted into a bypass chimney, i.e. past the waste heat steam generator. In the interests of highest possible system availability such a fast deactivation is however definitely not desirable.
In addition the currently prescribed adherence to the water level of the medium-pressure drum (MD drum) and low-pressure drum ND drum) above the minimum level during oil operation demands a complex inlet temperature control for the economizer of the high-pressure and medium-pressure system and for the condensate pre-heater. Changes to stationary states through different operating conditions in oil operation result in internal heat displacements in the waste heat steam generator which influence the heat acceptance of the medium-pressure and low-pressure evaporator. These can for example cause fluctuations in the drum water levels of the MD and ND drums and an undesirably high increase in pressure in the ND drum. To keep these fluctuations within the required operating limits, the amounts of water via the HD and MD economizer bypass valves must be subject to the appropriate superimposed control which demands an increased control effort.
Finally the currently demanded adherence to the minimum water level in the ND steam collection drum leads in the fully explained run mode sleeping mode” of particular interest as regards its basic concept, in which for example, during a rapid deactivation of the steam turbine the HD steam generated in the HD line is diverted via a bypass line directly into the condenser (bypass operation), whereas through an explicit pressure relocation and a shift of the heat emission and reception in the waste heat steam generator the production of MD and ND steam is to be bought to a halt, to additional costs as a result of an ND steam collection drum having to be dimensioned comparatively large. The fall in the water level in the ND drum with a rapid deactivation of the steam turbine is namely especially drastic here by virtue of the explicit accompanying pressure increase in the ND system. By contrast with the original alignment of the concept, a low-pressure diversion station, which reduces the fall in the water level during a rapid deactivation of the steam turbine, cannot therefore be completely dispensed with.